Ternary TiO2/P-GQDs/AgI nanocomposites with n-p-n heterojunctions for enhanced visible photocatalysis

TiO2-based materials are important photocatalysts but the rapid recombination of photoexcited electron–hole pairs, and the wide band gap both limit their photocatalytic activities. In this work, p-type phosphorus-doped graphene quantum dots (P-GQDs, 5~20 nm) were combined with n-type TiO2 nanoparticles (20~50 nm) and n-type silver iodide (AgI, 100~200 nm) by a facile sonication and precipitation process to yield a novel ternary photocatalyst TiO2/P-GQDs/AgI with n-p-n heterojunctions. The morphologies, structures, optical properties, and chemical environments of photocatalysts were all studied. The resulting ternary photocatalyst exhibited enhanced visible photocatalytic activity toward the degradation of methyl orange (MO), estimated to 70.4-fold that of pure TiO2. By contrast, the photocatalytic activities of TiO2/GQDs with an n-n heterojunction, TiO2/P-GQDs with a p-n heterojunction, and TiO2/GQDs/AgI (30%) with an n-n-n heterojunction showed 4.4-fold, 5.3-fold, and 14.2-fold higher than those of TiO2. The electrochemical and optical measurements revealed that the enhanced photocatalytic activity was linked to promoted charge carrier separation by n-p-n heterojunctions coupled with the increased absorption in the visible region. Overall, the new band structure connected in series p-n heterojunctions looks promising as a highly efficient photodegradation system for future use in decontamination of polluted water. Graphical abstract